The oxidative carbonylation of mono-olefins such as ethylene and propylene to prepare carboxylic acids and derivatives employing various catalyst systems, particularly noble metal catalysts is known; see for example, Fenton and Steinwand, Journal of Organic Chemistry, Vol. 37, 2034 (1972) as well as U.S. Pat. Nos. 3,397,226; 3,876,694; 3,907,882; 3,923,883; and 3,960,934.
In an article by Jiro Tsuji, Accounts of Chemical Research, Vol. 2, 144, (1969) and especially bibliographic references (36) and (37) noted therein, the carbonylation of butadiene-isoprene-palladium chloride complexes in alcohol to give 1,4-dichloro-2-butene and ethyl 3-pentenoate and ethyl 5-ethoxy-3-methyl-3-pentenoate and dimethylbutyrolactone, with other minor products is described. In a related article by S. Hosaka and J. Tsuji, Tetrahedron, Vol. 27, 2821-3829 (1971) the palladium catalyzed carbonylation in alcohol of various conjugated dienes and the reaction mechanism are shown.
While oxidative carbonylation reactions are generally known, the prior art does not show or describe the process of the present invention for the oxidative carbonylation of a diolefin, such as butadiene, to prepare an unsaturated diester which may be further processed by catalytic hydrogenation and catalyzed hydrolysis reaction sequences to prepare adipic acid and related derivatives. Hydrogenation of the unsaturated diester produces dialkyl adipates, e.g., dimethyl adipate which may be used as plasticizers and lubricants.
The process of the present invention is directed to the preparation of an unsaturated diester by the catalytic oxidative carbonylation of a diolefin. More particularly, the instant process relates to the synthesis of diesters by reacting carbon monoxide, oxygen, a diolefin such as 1,3-butadiene, isoprene, chloroprene and the like, at least a stoichiometric amount of an enol ether such as methyl vinyl ether or an alkoxycycloalkene such as 1-methoxycyclohexene and at least an equal molar quantity of an alcohol such as methanol based on the enol ether or alkoxycycloalkene, under elevated temperature and pressure conditions in the presence of a catalytic amount of a ruthenium, rhodium, palladium, osmium, iridium or platinum metal salt compound or mixtures thereof, and a copper (I), copper (II), iron (II) or iron (III) oxidant salt compound. Co-catalytic ligands or coordination complex compounds of the metal salt compounds while not required in the process of the invention, may also be employed.
The process of this invention provides an economic process for the preparation of a diester which may be an adipic acid precursor, by the oxidative carbonylation of a conjugated diolefin such as butadiene. There is provided a high conversion of the diolefin employed, especially 1,3-butadiene, and excellent yield selectivity to the diester. Carbonate esters, oxalate esters, carbon dioxide as well as other side reaction products associated with the oxidative carbonylation reaction are obtained in only trace amounts or eliminated by the reaction conditions employed in carrying out the process of the invention. The reaction is catalytic in both the platinum metal salt compound and oxidant salt compound and employs at least stoichiometric quantities of reactant diolefins, carbon monoxide, oxygen and/or air, and enol ether or alkoxycycloalkene as well as an equal molar amount of an alcohol based on the enol ether or alkoxycycloalkene. The reaction can be safely and conveniently carried out under a non-explosive oxygen or air/carbon monoxide atmosphere.